This invention relates to a microprogrammed computer, particularly a microprogrammed computer of the vertical type which control the changes of the function of each microinstruction and the execution sequence of microprograms.
A control apparatus such as a computer has a great number of logic gates. To simplify the control logic of such a control apparatus and improve the efficiency of the same, the microprogramming method is employed. As well known, the logic gates of the apparatus are controlled according to the execution sequence of microprograms. One of the processors controlled by such microprograms is shown in U.S. Pat. No. 3,646,522.
Hitherto two microprogramming methods have been applied to microprogrammed computers. One is the horizontal microinstruction method. In this method each instruction word is made relatively long, and its bits are made to correspond to the logic gates and serve to open and close them, respectively. The other is the vertical microinstruction method, wherein each instruction word is made relatively short and many bit data, which correspond to the instruction words, are decoded so as to control the logic gates of the apparatus.
However, either microinstruction method has a drawback. If the horizontal microinstruction method is to be applied, both the microprograms and hardware structure have to be complicated because the instruction words are long. As a result, it becomes difficult to prepare microprograms. If the vertical microinstruction method is to be applied, it is easy to prepare microprograms, but a decode circuit is indispensable. Moreover, in the vertical microinstruction method, the function of each instruction word is so simple and so limited that the number of program steps (or microinstructions) should increase, inevitably lengthening the microprograms themselves.